1. Field of the Invention
The present invention generally relates to navigation systems for movable bodies, and more particularly to an on-vehicle navigation system by means of GPS (Global Positioning System) receiver.
2. Background of the Related Art
There is a GPS (Global Positioning System) type navigation system as one type of navigation systems for movable bodies, such as men, vehicles, aeroplanes, ships, etc., which utilizes satellites. The GPS type navigation system generally receives electric waves emitted from three or more GPS satellites, and identifies the position of the receiving point by referring to pseudo-distance data including a time offset of a receiver based on the distance between each GPS satellite and the receiving point, and position data indicating each GPS satellite. Thus, GPS measurement data are outputted.
In this type of GPS navigation system, there is a system, in which a map matching is executed between the GPS position measurement data and map data stored in a recording medium, such as CD-ROM (Compact Disk-Read Only Memory), so that the current position and the traveling direction is displayed in the map on the display unit.
In this type of GPS navigation system, the map image displayed on the display unit is formed by use of, for example, map data issued by a national geography committee. Thus, it is easy for the user to understand the current position on the map or the positions of the main roads or railway stations on the displayed map image. However, in the actual cases, if there are narrow roads which are not shown in thus formed map, or if the vehicle is located at a place where large buildings disturbs the view of the user, the actual conditions around the current position can not be easily recognized, resulting in that the basic function of the navigation system can not be fulfilled.
In order to overcome this problem, such a method may be tried, in which the information as for the actual conditions, such as detailed information indicating areas of large buildings, orchard fields etc., is stored in advance, or the information as for all of the roads including narrow ones is stored in advance. However, this method has a basic problem that the data quantity gets very large, and is not practical.